The present invention relates to a process for the preparation of a regenerated cellulose fibre or yarn.
For greater clarity, the name xe2x80x9cyarnxe2x80x9d will be used for the products obtained by the process of the invention, this term denoting, for the requirements of the present description and for the assessment of the scope of the invention, products such as continuous yarns, fibres or rovings, with round or shaped cross sections and with any count.
Regenerated cellulose yarns have been produced for a very long time from a solution of a cellulose derivative such as xanthogenate, in a basic medium. This solution is spun with passage through a coagulation bath and then treatment to decompose the xanthogenic functional group and to regenerate the hydroxyl functional groups of the cellulose. However, this process, which uses carbon disulphide as xanthogenation agent, is very punishing to the environment and generates a great deal of effluent.
More recently, other processes have been provided, including the process known under the name Lyocell, which consists in dissolving the cellulose in an organic solvent, N.MMO (N-methylmorpholine oxide). The solvent is recovered after spinning in spin baths.
Provision has also been made for a process known as the xe2x80x9ccarbamatexe2x80x9d process, which consists in manufacturing a cellulose derivative, cellulose carbamate. This carbamate is obtained by reaction of cellulose with urea. The cellulose carbamate is subsequently dissolved in a sodium hydroxide solution and then spun at low temperature. The cellulose is regenerated by raising the temperature and giving off ammonia.
The latter process is currently in development (see article by A. Urbanowski published in xe2x80x9cChemical Fibers Internationalxe2x80x9d of September 1996, pages 260 to 262).
These novel processes, in particular the Lyocell process, require wet spinning. In addition, the fibres obtained by the Lyocell process have different technical characteristics from those of conventional cellulose fibres obtained by the xanthogenation process, as is described in the article by P. A. Koch which appeared in the journal Man-Made Fiber Year Book of September 1997, p. 41 to 47.
A particular aim of the present invention is to provide a process for the manufacture of regenerated cellulose yarns which respects the environment and which makes it possible to obtain yarns which are similar with regard to properties to Viscose or Rayon yarns obtained by the conventional carbon disulphide process.
Another aim of the invention is to provide a process for the manufacture of a regenerated cellulose yarn which makes it possible to carry out the spinning in a molten medium. The latter characteristic makes it possible to improve the profitability of the process as the melt-spinning rates are markedly higher than those of the dry or wet spinning processes.
To this end, the invention provides a process for the manufacture of a regenerated cellulose yarn which consists in spinning a solution of a cellulose derivative or the said cellulose derivative in the molten state through at least one die hole and in then regenerating the cellulose by treatment of the yarn obtained, characterized in that it consists:
in synthesizing a silylated derivative of the cellulose by reaction with a silylating agent,
in extracting the said silylated derivative of the cellulose from the synthesis reaction mixture,
in spinning the said silylated cellulose derivative, dissolved or brought to the molten state, through at least one die hole,
in treating the said yarn with a desilylating agent, in order to regenerate the cellulose and to recover a siloxane,
in regenerating the silylating agent from the siloxane recovered in the stage of regeneration of the cellulose.
According to the invention, the cellulose suitable for synthesizing the silylated cellulose can be of plant (wood, cotton, and the like) or animal origin. It can have a variable degree of polymerization DP, for example of between 100 and 5000. The DP of the cellulose is chosen according to the mechanical properties desired for the cellulose yarn to be manufactured.
Cellulose derivatives can also be used to synthesize silylated celluloses, in particular amorphous cellulose derivatives, which are more reactive, such as those substituted by organic radicals with a low degree of substitution DS (less than 1).
The term xe2x80x9cdegree of substitution DSxe2x80x9d should be understood as meaning the mean number of substituted hydroxyl groups per anhydroglucose unit. As each anhydroglucose unit comprises three accessible hydroxyl groups, the maximum degree of substitution DS is equal to 3.
In a preferred embodiment, the process applies particularly to the silylation of polysaccharides, in particular of cellulose, activated by treatment under pressure with ammonia and then explosion of the ammonia-impregnated polysaccharide according to the process disclosed in Patent Application WO 96/01274 or by reduction in pressure of the ammonia atmosphere as disclosed in Patent Application DE 19 51 10 61.
A compound for insertion or for inclusion between the polysaccharide fibres or chains can be added during the phase of activation by ammonia. Thus, this compound, added with the ammonia, preferably dissolved or dispersed in liquid ammonia, is uniformly distributed in the cellulose structure during the stage of pressure reduction or of explosion and keeps the polysaccharide chains separated from one another. The presence of this insertion compound renders the hydroxyl groups of the polysaccharide more accessible.
Mention may be made, by way of examples, as insertion compounds, of primary alcohols, secondary alcohols, phenols, ethers, acetals, ketones, xcex2-keto esters, amides, sulphamides, esters, urea derivatives, amino acids, steroids, mono-, di- or oligosaccharides and/or an aromatic compound comprising a heteroatom. The preferred compound of the invention is ethylene carbonate.
The process of the invention applies even more particularly to celluloses partially substituted by organic groups and more advantageously to cellulose esters or ethers exhibiting a degree of substitution DS of less than 1, advantageously of less than 0.7. These celluloses exhibit a low degree of crystallization, which renders the hydroxyl groups accessible.
According to a novel characteristic of the invention, the silylating agent corresponds to one of the following general formulae: 
in which:
n is between 0 and 20 inclusive
R1 which can be identical or different, represent linear or branched alkyl radicals comprising from 1 to 12 carbon atoms or aromatic radicals
R2 which can be identical or different, represent linear or branched alkyl radicals comprising from 1 to 12 carbon atoms or aromatic radicals
R represents an alkyl, aralkyl, aryl or alkylaryl radical or radicals of following general formulae: 
xe2x80x83in which:
R3, R4, R5, R7 and R8, which can be identical or different, represent the hydrogen atom or an alkyl group comprising from 1 to 4 carbon atoms
R6 represents an alkoxy group or an alkyl group comprising from 1 to 4 carbon atoms,
X represents a radical of following formula (V): 
xe2x80x83in which:
U represents a carbon, nitrogen, oxygen or sulphur atom,
T represents a carbon, nitrogen, sulphur or phosphorus atom,
V represents an oxygen, sulphur or nitrogen atom, and
T is other than U and than V.
According to another preferred characteristic of the invention, the silylation reaction is carried out in the presence of an organic swelling agent having a high dipolar moment which is advantageously higher than that of the alkoxy functional group of the silylating agent of formula (I). This swelling agent improves the accessibility of the hydroxyl groups of the cellulose. This swelling action is of use in particular when the cellulose has not been subjected to a prior activation treatment, such as activation by ammonia or substitution of a portion of the hydroxyl groups by organic radicals.
Mention may be made, as suitable swelling agents, of N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), N-methylmorpholine oxide (NMMO) or dimethylformamide, for example.
The cellulose/swelling agent ratio by mass is advantageously between 0.05 and 0.95, for example between 0.05 and 0.15.
However, the silylation process can be carried out with a cellulose/swelling agent ratio of between 0.15 and 0.95. In this case, it is advantageous to mix a portion of the silylating agent with the cellulose at a low temperature, preferably of between 20xc2x0 C. and 50xc2x0 C., in order to allow the silylating agent to spread into the structure of the cellulose, and then, in a second stage, to raise the temperature into the range indicated above and to add the remainder of the silylating agent. The first portion of silylating agent added can represent between 10% and 50% by weight of the total mass of silylating agent to be added.
According to another preferred characteristic of the invention, the silylation reaction is advantageously carried out in the presence of a catalyst, more particularly of a silylation catalyst, that is to say a compound with an acid, protic or Lewis-acid nature or a strong base. Mention may be made, as suitable catalyst, by way of examples, of para-toluenesulphonic acid, the pyridinium salt of para-toluenesulphonic acid, trifluoroacetic acid, para-trifluoromethylbenzenesulphonic acid, trifluorosulphonic acid, hydrochloric acid, ferrous or ferric chlorides, tin chlorides or pyridine.
The amount of this catalyst is not critical and corresponds to a catalytically active amount. By way of example, the amount of catalyst is between 0.1 and 5% by weight with respect to the reaction mass. The catalyst is advantageously used with the silylating agents of formula (I).
On the other hand, the silylation reaction can be carried out without catalysis with the silylating agents of formula (IV). This absence of catalyst can be highly advantageous when the silylated cellulose has to be brought to high temperatures during its use or treatment.
In one embodiment of the invention, the silylation reaction is advantageously carried out at a temperature of between 100xc2x0 C. and 150xc2x0 C., preferably between 120xc2x0 C. and 150xc2x0 C. This temperature is advantageously determined in order to carry out the reaction with distillation of the alcohol formed. The silylating agent is added all at once to the reaction mixture.
In another embodiment, a first portion of the silylating agent, representing between 10 and 50% by weight of all the silylating agent to be added, is brought into contact at a low temperature, advantageously of between 20xc2x0 C. and 50xc2x0 C., with the cellulose to be treated. After maintaining at this low temperature for a certain time, in order in particular to allow the agent to spread into the structure of the cellulose, the mixture is heated to a temperature of greater than 60xc2x0 C., advantageously of between 60 and 100xc2x0 C., the remainder of the silylating agent being added to the mixture.
The desired degree of substitution (DS) can be the maximum degree, that is to say 3. However, the process of the invention makes it possible to obtain silylated cellulose compounds exhibiting advantageous properties for a degree of substitution of less than or equal to 3 and preferably of between 1 and 2.5.
The desired degree of substitution can be obtained by controlling either the conditions of duration, temperature and pressure of the reaction or the molar ratio of the silylating agent to the number of cellulose hydroxyl groups. Thus, this ratio will be at least equal to the stoichiometric ratio determined according to the desired degree of substitution. This ratio will preferably be less than 15 times the stoichiometric ratio, calculated with respect to the hydroxyl groups to be silylated.
The silylating agents of general formula (I) which are suitable for the invention are more particularly alkoxysilanes, such as n-butoxytrimethylsilane, tert-butoxytrimethylsilane, sec-butoxytrimethylsilane, isobutoxytrimethylsilane, ethoxytriethylsilane, octyldimethylethoxysilane or cyclohexanoxytrimethylsilane, or alkoxysiloxanes, such as butoxypolydimethylsiloxane.
These silylating agents can advantageously be produced by reaction of an alcohol, such as n-butanol, isobutanol, 2-butanol or cyclohexanol, with a disiloxane, such as hexamethyldisiloxane, in the presence of an acid catalyst, such as para-toluenesulphonic acid.
This preparation of the silylating agent by reaction of an alcohol corresponding to the alkoxy radical of this agent with the disiloxane comprising the silane portions of the silylating agent makes it possible, in the process of the invention for the manufacture of regenerated cellulose yarn, not to consume silylating agent or, as at the very least, to limit this consumption. Thus, the process of the invention is highly economical.
This is because, during the reaction for the silylation of the cellulose, the alcohol formed is advantageously extracted from the reaction mixture by distillation and recovered. Furthermore, the regeneration of the cellulose results in the production of a disiloxane comprising the two silane or siloxane units, grafted beforehand onto the cellulose, connected to one another via an xe2x80x94Oxe2x80x94 bridge. The silylating agent will be resynthesized by the action of the recovered alcohol on this disiloxane.
The silylating agents of formula (IV) are advantageously obtained by reaction of a compound chosen from the group consisting of SO2, SO3, CO2, P2O5, CH2xe2x95x90Cxe2x95x90O and HCNO with a disiloxane, such as hexamethyldisiloxane.
The extraction of the silylated carbohydrate from the reaction mixture can be carried out by several processes, including filtration, centrifuging, precipitation or distillation processes. The silylated compound extracted is advantageously washed with water and solvents, such as acetone, and then dried.
The degree of silylation of the compounds obtained is determined by measuring the increase in weight of the cellulose. This measurement can be corroborated by NMR analysis or quantitative determination of the alkylsilyl units present in the carbohydrate by gas chromatography.
According to the process of the invention, the silylated cellulose is used as starting material for the manufacture of cellulose yarn either by spinning a solution of this silylated cellulose or by melt spinning when the latter exhibits a softening or melting point of less than 350xc2x0 C., for example of between 200xc2x0 C. and 300xc2x0 C., preferably of less than 260xc2x0 C.
The term xe2x80x9cmelting or softening temperaturexe2x80x9d should be understood as meaning the temperature at which the silylated cellulose exhibits a melt flow index compatible with melt-spinning processes.
In the case of spinning a silylated cellulose solution, the silylated cellulose, after extraction from the synthesis reaction mixture, is dissolved in a solvent chosen from the group consisting of N-methylpyrrolidone, dimethylacetamide, dimethylalkylureas, such as dimethylethylurea, formamide, dimethylformamide, tetrahydrofuran, dimethyl sulphone, tetramethylurea and tetramethylfuran, for example.
The concentration of cellulose should be as high as possible for better productivity of the process.
After passing through the die, the solvent can be evaporated (dry spinning). According to another embodiment, the fibre can pass through a coagulation bath, which brings about precipitation or coagulation of the silylated cellulose and extraction of the solvent (wet spinning).
In the case of melt spinning the silylated cellulose, the spinning temperature is advantageously greater than the melting temperature by 5xc2x0 C.
After melt, dry or wet spinning, the yarn produced is treated with a regeneration medium. This treatment can be carried out by immersing the yarn in or passing the yarn through a regeneration bath. This bath can be a water/alcohol mixture comprising an acid for bringing about desilylation of the cellulose and its regeneration.
The yarn is subsequently washed and dried.
The regenerated cellulose yarn thus obtained can be subjected to all the conventional treatments used in the manufacture of synthetic yarns.
Mention may be made, by way of examples, of drawing, texturing, crimping or relaxing processes. It is also possible to deposit, on its surface, sizing agents for modifying its surface properties, such as hydrophilicity, hydrophobicity, stain-repellency properties or lubrication, for example.
Of course, these yarns can also be dyed.
The regenerated cellulose yarns obtained by the process of the invention are used in particular in the form of continuous yarns or of fibres for the manufacture of woven or knitted textile surfaces or of nonwovens. They are also of use as yarns for reinforcing structures made of synthetic material, such as rubbers. Thus, they are used in reinforcing tyres.